1. Field of the Invention
This invention relates to designing, building and maintaining a multivariable model predictive controller, specifically relating to creating a universal, consistent and robust design for a process operation in general.
2. Background of the Invention
Model Predictive Control (MPC) has been in use in industry since early 1980. It forms the backbone of advanced process control in chemical plants, refineries and other process industries. MPC refers to a class of algorithms that compute a sequence of future manipulated variable adjustments in order to minimize the future response of complex multivariable processes. MPC performs real time optimization and control of simple to complex processes. A MPC controller employs models of one form or other of the process to predict the effect of past changes of manipulated variables and measured disturbances on the output variables under control. The system dynamics are described by an explicit one to one model of the effect on the controlled variable to a unit change in the manipulated variable. A number of different mathematical forms can be used to represent the process effects. Process input and output constraints are included directly in the problem formulation so that future constraints violations are predicted and prevented.
Since its inception, MPC has been widely applied practically to every sector of process industry worldwide with the number of applications approaching in thousands. This technology has been applied aggressively over last 30 years with demonstrated success. However, there has been an undercurrent of weakness of this technology that is increasingly being now recognized. Most if not all of these applications are being re-done, re-engineered and re-deployed. In some cases, the same application is re-engineered to work around current problems only to re-invent the past solutions. In other words, these applications are hoping from one design to next depending on who is doing the project. This problem of inconsistency of design and consequent re-work is wide spread. This is happening even within a large oil company where for instance, for a fluidized catalyst-cracking unit (FCC unit), one will find as many different designs as there are the number of FCC units. To make the matters worst, the same FCC unit will be re-worked with different design three times over and still would not have a robust design.
By now, most large companies have settled on selection of a particular MPC technology available form a number of control vendors such as Honeywell, AspenTech etc and all these re-work is primarily not so much with any change in the MPC technology but more to do with re-formulating of the unit controller configuration based on redressing the past performance shortcomings. Each time this is done, a new design of MPC evolves.
In the prior art there is no method of design that would produce a consistent design of MPC for the same process. Consequently, the experience and knowledge gained from the previous MPC application is hard to retain with a new MPC application even on the same process unit. There is a tendency to re-invent the past solutions and in the process unintentionally either re-create old problems or create new problems.
The simple question is why is that after 25 years of experience of implementing FCC MPC across over 300 refineries worldwide that in the industry there are articles being written about FCC APC. The question is, is it that the FCC unit operation is too complex that it would defy any attempts to control it well. Or, is it that there is something lacking in the application of MPC technology that is not allowing this unit operation to be tamed. Or both. Given the wealth of understanding of the FCC process and its operation, it is implausible that the lack of understanding of the process could be the reason for this problem. Instead, it is plausible that the methodology of application of MPC is not directly and intimately tied to the process knowledge. Or how else one can explain the differences in the design of MPC for FCC. After all, it follows logically that for same process the design of APC should be same. It is not the case that an FCC APC is designed with no understanding of the process. In fact, every APC begins with understanding process flow chart, and detailed discussion of unit operation with the operator and the control engineers. So, then why is that there are still significant differences in FCC APC design? The answer lies in the absence of a method of design of process control that is based fundamentally on the process relationship. The method of design commonly practiced in the industry is at best can be described as an ad hoc process. This invention highlights the lack of structure and consistency of design of an example FCC APC based on the method commonly used and contrast it with a method of control design based on Core Process Relationships.
Numerous unsuccessful attempts have been made by practitioners in the field to address specific issues and problems relating to certain aspects of MPC such as model identification. However, in the prior art the basic form of model used since its inception in 1980 still remains the same. Recently, an alternate form of model based on control valve output in place of customary based on set point has been proposed (U.S. Pat. Nos. 6,980,938, 7,263,473 and 7,447,554) but it only offers a partial solution to mitigating the problem of re-engineering MPC.
U.S. Pat. No. 7,187,989 by the inventor is incorporated by reference into this application in the entirety. This invention addresses the aforementioned issues and describes a methodology for specifying, designing and operating a universal robust model predictive controller (U-MPC). The U-MPC methodology permits a designer to sort out the intrinsic core process relationships from the control valve actuation relationships in order to build a robust controller design that can be implemented in a variety of method of control valve actuations. Based on the separation of the core process relationships and the control action relationships, a new method of model predictive control is presented based on manipulated variable process value based process models in place of the customary control models based on either manipulated variable set point or manipulated variable controller valve output. By definition and design, the PV-based process models used in the U-MPC are devoid of any form of controller dynamics be it PID based or any other form of regulatory controller. The U-MPC basically performs optimization and control in terms of the core process relationships entirely independent of method of control actuation but at the same time relate back to the method of control actuation in any form that may include a cascading regulatory controller or direct control valve actuation. Thus, the method of control actuation can be preserved for what it does best in accordance with the control design considerations. That is, the regulatory controllers can be designed for what is best in regard to disturbance rejection and closed loop performance independently of the optimization and control of the process unit as a whole. In other words, it is not necessary to give up on the regulatory controllers to make the control models devoid of their dynamics and then attempt to do the same disturbance rejection with the process unit model predictive controller.